Recrystallization of pyrolytic boron nitride

ABSTRACT

A METHOD FOR STRUCTURALLY TRANSFORMING PYROLYTIC BORON NITRIDE INTO A PRODUCT CLOSELY RELATED TO A SINGLE CRYSTAL HEXAGONAL BORON NITRIDE STRUCTURE BY SUBJECTING A DISC OR PLATE OF THE STARTING MATERIAL TO A SIMULTANEOUS TEMPERATURE OF AT LEAST 2250*C. AND PRESSURE OF 5000 P.S.I. IN A NON-REACTIVE ATMOSPHERE. THE PRESSURE IS APPLIED UNIAXIALLY IN THE PREDOMINANT DIRECTION OF THE C-AXES AND APPLIED SO AS TO PLACE THE MATERIAL IN COMPRESSION.

May 11,119.71 A. w. MOORE 3,578,403

RECRYSTALLIZATION 0F PYROLYTIC BORON NITRIDE Filed July 5. 1968 1 FIG. 2.

W E BY 5. mm Q ATTORNEY United States Patent O 3,578,403 RECRYSTALLIZATION F PYROLYTIC BORON NITRIDE Arthur W. Moore, Parma, Ohio, assignor to Union Carbide Corporation Filed July 5, 1968, Ser. No. 742,614

Int. Cl. C01b 21/06 US. Cl. 23-191 6 Claims ABSTRACT OF THE DISCLOSURE A method for structurally transforming pyrolytic boron nitride into a product closely related to a single crystal hexagonal boron nitride structure by subjecting a disc or plate of the starting material to a simultaneous temperature of at least 2250 C. and pressure of 5000 p.s.i. in a non-reactive atmosphere. The pressure is applied uniaxially in the predominant direction of the C-axes and applied so as to place the material in compression.

FIELD OF INVENTION This invention relates to a method of producing a structurally well defined form of pyrolytic boron nitride.

Pyrolytic boron nitride is generally prepared by the thermal decomposition of boron trichloride and ammonia vapors on a graphite substrate at a temperature of about 1900 C. A complete description of this process is set forth in US. Pat. 3,152,006. Pyrolytic boron nitride produced in this manner is characterized by a density of between 2.02.2 grams/cubic centimeter and a crystallite size of 50-l00 angstroms. The material is further, characterized by a high degree of preferred orientation of crystallites and the basal planes tend to be aligned parallel to the substrate surface. The crystallite preferred orientation is defined as the full Width at one-half maximum intensity of the X-ray 002 (basal plane) reflection and for this material it measures between 60 and 100'".

While pyrolytic boron nitride has been useful in certain applications, structural modifications of this material to improve certain properties would be most desirable.

SUMMARY OF THE INVENTION The present invention provides a process whereby a major structural transformation of pyrolytic boron nitride is effected. By subjecting pyrolytic boron nitride to a temperature above 2250 C. and simultaneously applying a uniaxial pressure of between about 5000 pounds per square inch and about 15,000 pounds per square inch in a direction perpendicular to the basal planes, the pyrolytic boron nitride in transformed into a soft, lustrous, transparent material. The end product is characterized by a theoretical density, i.e. 2.28i0.0l grams per cubic centimeter, is highly crystalline and has a crystallite preferred orientation of about 2. The material is further enhanced by a wall defined solid structure closely related to that of single crystal hexagonal boron nitride. These important properties enable the stress recrystallized pyrolytic boron nitride to be used in microwave devices such as a polarizer or an infrared window or even as a monochromator for X-ray reflection.

In the preferred manner of practicing the invention, pieces of pyrolytic boron nitride are machined into discs or plates with substantially parallel upper and lower surfaces. These surfaces are representative of the predominant direction of the basal planes of the crystallites. One or several of the discs are positioned between end plates composed of hot pressed boron nitride and the components are placed in a thick-walled cylinder of the same material. The assembly is placed in a graphite furnace such that uniaxial pressure is transferred to the pyrolytic boron 3,578,403 Patented May 11, 1971 nitride by graphite plungers. The temperature is raised to 22502450 C. and the pressure is then increased to between 3000 and 6000 p.s.i. These conditions are preferably maintained from about A2 to about 5 hours. Upon cooling, the pressure is released to about one-fourth of its maximum value until room temperature is reached and the sample is then removed from the furnace.

Alternatively, one or several discs of pyrolytic boron nitride are placed between two plates of pyrolytic graphite of larger diameter, and the assembly is placed in a graphite die in a graphite hot-pressing furnace. The temperature is raised to 22502450 C., then the pressure is increased to between about 5 000-15 ,000 p.s.i. These conditions are also maintained for about M to about 5 hours.

DESCRIPTION OF THE DRAWING The invention will be more fully described with reference to the drawing wherein:

FIG. 1 is a cross sectional view of part of a vertical induction furnace which is useful in the process of the invention; and

FIG. 2 is another embodiment of furnace equipment which can be used in the process of the invention.

Referring to FIG. 1, a cylindrical induction furnace 10 (indicated as having segments 12, 14 for illustrative purposes) is preferably composed of graphite which acts as a susceptor for the inductive heating means (not shown). Graphite plungers 16, 18 are positioned within the furnace 10 and adjacent the pyrolytic graphite plates 20, 22. A disc 24 of pyrolytic boron nitride is placed between plates 20, 22. In operation of the furnace, pressureis applied along the longitudinal axis of the plungers through the plates 20, 22 to compress the disc 24 while heat is supplied to the disc. The disc is initially positioned such that the majority of the C-axes (the direction perpendicular to the plane of deposit during formation of the pyrolytic boron nitride) of the crystallites are parallel to the direction of the applied force (as indicated by the arrows in FIG. 1). This initial orientation of the specimen must be effected for the structural transformation to be successful. Furthermore, for proper results, the uniaxial pressure should be simultaneously applied to the entire area of the proper surface of the pyrolytic boron nitride, and the pressing device in immediate contact with the boron nitride during processing, such as plates 20, 22, should be larger in diameter than the material being treated so that continued uniaxial pressure can be maintained even during the expansion of the material along the basal plates, i.e. in the direction perpendicular to the applied force.

FIG. 2 illustrates a variation in the equipment which can be used in the process of the invention. As there shown, a prepacked assembly comprising a graphite sleeve 26 which encloses a boron nitride cylinder 28 and graphite plungers 30, 32 is prepared for insertion as a unit into a furnace. The assembly further includes hot pressed boron nitride end plates 34, 36 and a pyrolytic boron nitride disc 38 positioned between the plates. Pressure is applied through the plungers and plates to the pyrolytic boron nitride in the direction indicated by the arrows on the plungers 30, 32. The predominant C-axes direction of the crystallites within the pyrolytic boron nitride being treated is again aligned in the direction of applied pressure as shown by the arrow alignment on the material. The use of the prepacked assembly facilitates handling and cooling operations during the process and minimizes processing time.

The pyrolytic boron nitride starting material should be substrate nucleated, it having been determined that continuously renucleated pyrolytic boron nitride does not readily structurally transform when subjected to the aforementioned processing conditions. By substrate nucleated 3 pyrolytic boron nitride is meant material free of codeposited gas-phase formed particles which act as new nucleation sites. These nucleation sites, apart from representing a structural defect, cause locally misoriented pyrolytic boron nitride regions to form which inhibit the mately 9% in diameter. The structural transformation was similar to that in the previous examples while a crystallite preferred orientation of between 2.45 and 2.75 was measured across the sample.

A number of other samples similar to those specified in annealing of the whole sample. In addition to this requirethe examples were processed in equipment similar to that ment, the starting material should be free of delaminations shown in the drawing at pressures between 5000 p.s.i. and and of the highest preferred crystallite orientation for 15,000 p.s.i. for /2 to 1 hour. The results of the tests are best results to be achieved. shown in the following table:

TABLE I Starting material (PBN) End product Crystailline1 Crystallinea G e 'I '8 erre Density, Ton1p.(C.) of deposit and oi'iiitatioii, Density, ori ination, Structural Batch No g./cc. microstructure degrees g./cc. degrees transformation 1 2.13-2.13 LEE-T0050; ivory color bands, substrate 63 2.26 3.9 Tranlsjfondneid at 2,350 0,8,000 p.s.i.

n I: an 0 I'QglO S. 2 2.20-2.20 1875-2 0 0; white, continuously renucle- 74 2.24-2.27 13-40 to dtizagfoormation at a .an p.s.i. 3 2.07-2.14 1572-1900;d slightly off White, substrate 60 2.28 2 somsafilrshes transformed at 2,400

c eatc p.s.i. 4 2.08-2.10 cy igl eld Y1),8?1(,18715; \Yhitiehsubstrate nuclc- 76, 06, 97, 101 2. 24 13-23 Veryjlittlza1 irfi'ahsormation at 2,400

, a a 1 ae. .an p.s.i. 5 2.10-2.12 15375 -1000: wh ite s i ibstrate nucleated, 61 2.27 3.7 Readig otransformed at 2,300 0-- 6 2. 03-2. 09 l,s7% l i %i) ivory color bands substrate 58, 61, 75, 96 2.28 1.9 Do.

nucleated, coarse grain, delaminated at ma or CODES. 7 2.11-2.13 chcleg d1,825 -1,8751; lllaaldlyffdelllaitminated, 94 N0 measurements None at 2,450 C.

0 1 0 W l 0. 8 2.10-2.12 nszigmiffyiii, sub s trate nucleated, 70 2.27 3.5 Rgagigyral iglg ned at 2,300 O.-

a' acoe. p.s.i. 9 2.09-2.12 1,87 5-1 d( lightly off white, substrate 53, 62, 76 2.275 2. 2 Readily transformed at 2,350 C.-

nucleated, delaminated at cones. 2,400 0., 7,500 p.s.i.

The following examples set forth several modes of The process of the invention is carried out in a nonpracticing the method of the invention. reactive atmosphere and could be practiced in an atmos- EXAMPLE I phere free of carbon. Certain refractory borides such as A disc of substrate nucleated pyrolytic boron nitride T1132 and .ZrBZ can Serve heatmg elements #3 [they measuring 5/8 inch in diameter and inch thick and rnodynamrcally stable with respect to boron n1tr1 e in a ing a density of 2.07 grams per cubic centimeter was mtrogen atmosphere the tempeiraglres f f g g' placed between two hot pressed boron nitride plates which g l of g; W boron. e so 6 were 1 inch in diameter. A /2 inch thick boron nitride 't y latlon Sue Wu m a so ar umace n addition, high pressures of nltrogen could be used to supcyhnder enclosed the plates and disc as shown in FIG. 2 4,0 and the assembly was placed in a furnace with a graphite press.decol.nposltlon. of the pyrolytlc bPrOn and to slevc having a wall thickness of /8 inch. The disc was perm i mcr.eaSe i temperature dunng processmg' inductively heated to 2400 C. and a uniaxial compressive What IS clalmed pressure of 5500 p.s.i. was applied to the disc through A .method for .lmprovmg. i preferlefl crysta graphite plungers in the direction of the Oaxes of the orlentation of pyrolyt1c boron n1tr1de compnslng the steps crystallites. The simultaneous temperature and pressure h b r tm were maintained for 1 hour. The material was transformed a h ea 11% tn n1 glte m a ggntetgsg :1: olsd into a soft, lustrous, transparent, flaky material having a g i F? f i n f th density of 2.28:0.01 grams per cubic centimeter and a 3 b W f i a 10 o e on crystallite preferred orientation of 2 as compared to 60 m n 6 G reonen e in the Starting material (b) applying simultaneously a uniaxlal pressure hetween about 3000 pounds per square inch and, about EXAMPLE II 15,000 pounds per square inch to the entire area of at least one of the surfaces of said boron nitride in A dlsc of substrate nucleated pyrolytic boron mtride a direction substantially parallel to the predominant having a diameter of inch, a thickness of 4 inch and Oaxes crystallite orientation; a density of 2.08 grams per cubic centimeter was placed (C) Continuing the application of said pressure and between 1 inch diameter pyrolytic graphic plates in a said heat for a period of time between about graphite hot pressing furnace. The temperature of the disc and about 5 hOurs and then was increased to 2325 C. and a uniaxial pressure of (d) removing said pressure a said heat from the 7500 p.s.l. was simultaneously applied for /2 hour. The pyrolytic boron nitride disc was caused to shrink 22.3% in a direction parallel to 2 The method of claim 1 wherein prior to the apphca the alpphed force (5 the Ones) and expand 211) tion of said temperature and pressure there is added the PrPX1ma}telY 10% dlameter- The crystanlte step of smoothing those surfaces of said pyrolytic boron orientat on was 2. l and the structural transformation nitride to which Said pressure is applied was sl'mllar to that Example 3. The method of claim 1 wherein said boron nitride is EXAMPLE In it plate hashing1 substantially parallel1 upper and lower suraces to w ic said pressure is app ie A substrate nucleated pyrolytic boron nitride disc hav- 4. The method of claim 3 wherein said plate has a ing a diameter of 1.70 inches, a thickness of 4 inch and thickness between about inch and about A inch. a density of 2.09 grams per cubic centimeter was placed 5. The method of claim 1 wherein at least two discs between 3 /2 inch diameter plates of pyrolytic graphite in of pyrolytic boron nitride are placed between two plates a graphite hot pressing furnace and processed at 2300 C. of pyrolytic graphite having a larger diameter than the and 10,000 p.s.i. for 45 minutes. The disc was thereby boron nitrides discs prior to the application of pressure caused to shrink 21.7% in a direction parallel to the and heat. applled force (along the C-axes) and to expand approxi- 6. The method of claim 1 wherein the pressure in 6 step d is released to about one-fourth of its maximum 3,212,851 10/1965 Bundy et a1. 23-491 value until room temperature is reached. 3,212,852 1 0/ 1965 Bundy 23--191 3,351,690 11/1967 Stover 23--191X References Cited UNITED STATES PATENTS 5 OSCAR R. VERTIZ, Primary Examiner 2 0 314 10 1957 Taylor 23 191 RODMAN, Assistant Examiner $152,006 10/1964 Basche 23-191X 

